1. Technical Field
This invention relates to the process for preparing stabilized wholly aromatic polyester, also termed polyarylate, compositions. More specifically, this invention relates to a method for producing a stable, particularly hydrolytically stable, high molecular weight, wholly aromatic polyester, hereinafter referred to as polyarylate, by reacting a polyarylate resin that melts or flows below about 475.degree. C., preferably below 375.degree. C. in the melt, with an aromatic carbonate capping agent thereby reducing the terminal carboxyl acid group content in the polyarylate resin. The term aromatic carbonate includes diaryl carbonates, unsubstituted polyarylcarbonate oligomers and polymers, and substituted polyarylcarbonate oligomers and polymers.
Polyesters are produced from polycarboxylic acids and polyhydric alcohols. When both the polycarboxylic acid and the polyhydric alcohol are aromatic in nature, the resultant polyester is wholly aromatic and is referred to as a polyarylate. In the case of either polyarylates or aliphatic polyesters, there often remains, after reaction between the polycarboxylic acid and the polyhydric alcohol, some free carboxylic acid groups on the ends of the polyarylates or the aliphatic polyesters. This is particularly true when the polyarylates are synthesized via a melt polymerization method.
It is well known in the trade that free carboxyl acid end groups are detrimental to the stability of a polyarylate. For example, hydrolytic stability, high temperature stability, and stability during reprocessing, recycling, and reclaiming procedures all decrease as the terminal carboxyl acid group content of the polyarylate increases. As such, when the terminal carboxyl acid group concentration in the polyarylate is decreased, the stability of the polyarylate will be increased. In the present invention, there is developed a novel process for reducing the terminal carboxyl acid group content on a polyarylate. This process involves reacting a polyarylate, in the melt, with an aromatic carbonate, particularly a diaryl carbonate, an unsubstituted polyarylcarbonate polymer or oligomer, or a substituted polyarylcarbonate polymer or oligomer. The aromatic carbonates act as end-capping agents for the terminal carboxyl acid groups on the polyarylate, thereby resulting in a polyarylate having a reduced concentration of carboxyl acid end groups. When the aromatic carbonate is a brominated polyarylcarbonate oligomer, not only is the terminal carboxyl acid group concentration for the polyarylate decreased but also the flame resistance of the polyarylate is increased. The end-capped polyarylates of the present invention, as produced by the process of the present invention, are useful wherever there is needed a polyarylate with reduced terminal carboxyl acid group concentration having improved stability, particularly improved hydrolytic stability or improved stability upon subsequent reprocessing, reclaiming, and recycling. The polyarylates end-capped by the process of the present invention are useful in wire and cable applications and other engineering applications.
2. Description of Related Art
Much research has been directed towards developing a means by which to end cap the residual carboxyl acid end groups of polyesters. This research is disclosed in the patents or publications described below.
U.S. Pat. No. 2,863,854, granted Dec. 9, 1958 to Wilson, discloses a method of lowering the acid numbers of polyester materials, said polyester materials being made from dibasic acids and aliphatic polyhydroxy compounds, by reaction with alkylene carbonates. U.S. Pat. No. 3,300,447, granted Jan. 24, 1967 to Thoma, discloses a process for reducing the acid number of polyesters made from polycarboxylic acid and aliphatic polyhydric alcohol by reaction with a compound which contains at least two groups having the formula ##STR1## where R is a lower alkyl radical. While all of the above references disclose some method to reduce the number of terminal acid groups on the polyester, none disclose the particulars of the present invention with regard to the capping of aromatic polyesters in the melt by use of aromatic carbonates.
U.S. Pat. No. 3,752,866, granted Aug. 14, 1973 to Doerr, discloses a process which provides for the late addition of a polycarbonate to a fiber forming polyester melt, said polyester being made from diacids and aliphatic diols, which results in a polyester having a lower level of carboxyl end groups. There is no mention of wholly aromatic polyesters.
U.S. Pat. No. 4,327,207, granted Apr. 27, 1982, to Lazarus, discloses high molecular weight linear condensation polyesters stabilized against deterioration by reaction with alkylene carbonates in the presence of a catalytic amount of sodium iodide or potassium iodide. U.S. Pat. No. 4,348,314, granted Sept. 7, 1982, to Lazarus, discloses the same as the preceding patent except that the catalyst is selected from the group consisting of sodium thiocyanate and potassium thiocyanate. Neither reference discloses a method for end-capping polyarylates, nor is there disclosure of aromatic carbonates as end-capping agents.
U.S. Pat. No. 4,462,947, granted July 31, 1984, to Huggard, discloses crystalline polyesters that are made from aromatic dicarboxylic aCids and glycols and that are foamed with an aromatic polycarbonate. There is no disclosure of high molecular weight, wholly-aromatic polyesters.
U.S. Pat. No. 4,533,702, granted Aug. 6, 1985 to Freitag, discloses a method for producing aromatic polyester carbonates having increased notched impact strength and made from solutions containing aromatic polyester and aromatic polycarbonate. This reference employs a solution polymerization process and further, it is directed towards the direct synthesis of a copolymer and towards a process for producing an aromatic polyester carbonate with improved toughness rather than towards a process for end-capping the terminal carboxyl acid groups on a polyarylate.
U.S. Pat. No. 4,436,894, granted Mar. 13, 1984, to Urasaki et. al., discloses a wholly aromatic copolyester composed mainly of isophthalic acid units and hydroquinone units, said polyester having a terminal carboxyl group concentration of not more than 90 meq/10.sup.6 g There is further disclosed a process for producing a wholly aromatic copolyester having a reduced viscosity of less than about 0.6 by polycondensing in the presence of a polycondensation catalyst a mixture consisting of isophthalic acid, hydroquinone, a diaryl carbonate, and at least one other compound. This is a phenyl-ester based polymerization and the process disclosed focuses on synthesizing the polymer, not on end-capping a finished polyarylate. Further, the diaryl carbonate is used to provide the source of phenol for the phenyl-ester based polymerization and it is not used as an end-capping agent.
U.S. Pat. No. 4,137,221, granted Jan. 30, 1979, to Hara et. al., discloses a heat-curable ester group containing polymer composition prepared by reacting a linear aromatic polyester with a diaryl carbonate to depolymerize it to a low molecular weight polyester and either mixing it or partially reacting it with a monomeric or solvent-soluble polymeric compound containing at least two hydroxyl and/or secondary or primary amino groups. In this reference, the diaryl carbonate acts to depolymerize the linear aromatic polyester while in the present invention, it is used to cap the free acid ends on the polyarylate without significantly reducing molecular weight.
U.S. Pat. No. 4,680,372, granted July 14, 1987, to Rosenfeld, discloses a process for preparing an end-capped aromatic polyester by melting together a mixture of a dicarboxylic acid or diester thereof, a bisphenol and phenyl benzoate and reacting the mixture at elevated temperature. There is no disclosure of the particular end-capping agents used herein.
While it has been cited that end-capping of the terminal carboxyl groups on polyesters is well recognized as a method for improving polymer stability and that several techniques exist for the melt-capping of aliphatic polyesters or solution capping of aromatic polyesters, none of the above references disclose the particulars of the present invention with regard to capping the carboxyl acid end groups of polyarylates in the melt by use of aromatic carbonates.